The goal of our project is to understand processes of family formation in relation to career trajectories among young adults, and the consequences of family formation behaviors for health outcomes in young adulthood. With four waves of Add Health data, this project examines how early experiences and investments in romantic relationships and education during adolescence initiate trajectories that extend into adulthood and influence key decisions about family formation and career development in the 20s. Unique data that are not available in any other national data set are used, including involvement in romantic relationships during adolescence and new Add Health educational data on curriculum and academic intensity from high school transcripts. We first document patterns and differentials of family formation and career development among young adult men and women and across socioeconomic, race, ethnic, and immigrant groups. With a life course perspective, we draw on theories of socialization and opportunity structures to define the important mechanisms of influence within the three domains of context, biology, and behavior that operate in family formation trajectories. Their effects are then integrated in a dynamic longitudinal model of the determinants of family formation in young adulthood, including marriage, cohabitation, and marital and nonmarital childbearing. We use innovative methods to analyze the simultaneous decisions involved in processes of family formation and contribute to knowledge about how adolescent experiences and development are linked to behavior in young adulthood. Finally, we examine key hypotheses about the impact of union status and early nonmarital childbearing on mental health and stress outcomes, exploiting new biological data at Wave IV, as well as repeated measures of mental health over the four interview waves in Add Health.